A coreless linear motor is a linear motor of a type in which a coil is not wound around the core, that is, an armature does not have a core. Such a coreless linear motor has advantages that there is no cogging, a thrust fluctuation is small, precise control is easy, and so on.
Coreless linear motors are used in various fields, for example, machine tools, injection molding machines, and semiconductor production apparatuses.
In the coreless linear motor disclosed in Patent Document 1, Japanese Patent Publication (A) No. 2002-165434, a holding plate to which a coil is fixed is arranged between a pair of rows of permanent magnets arranged in straight lines. The interaction of the magnetic flux heading from one row of permanent magnets to the other row of permanent magnets and current flowing in the coil generates thrust based on Fleming's left-hand rule. The movable member of this coreless linear motor has a low rigidity since the coil does not have a core. In order to raise the low rigidity, stainless steel, FRP (fiber reinforced plastic, registered trademark of DuPont Corporation), or another non-magnetic material having a high rigidity is formed flat on the two surfaces of the holding plate and the coil is fixed to this by a resin in order to secure the rigidity of the movable member. However, in the coreless linear motor having the above structure, the holding plate of the coil is present in the path of the magnetic flux of the magnetic circuit, therefore the holding plate cannot be made thick. Namely, in the coreless linear motor of the type fixing the coil to the two surfaces of the holding plate, the greater the thickness of the holding plate, the lower the efficiency of utilization of the magnetic flux generated by the permanent magnets, therefore the holding plate cannot be made thick and there is a limit to improvement of the rigidity of the movable member in its structure. In this way, when a sufficient rigidity cannot be secured in the holding plate, there is the disadvantage that vibration is easily generated when the coreless linear motor is driven and the control loop gain cannot be raised.
In the coreless linear motor explained above, the coil constituting the armature is fixed to the holding plate by a resin having a low heat conductivity and the holding plate is formed by stainless steel or another material having a low heat conductivity, therefore the temperature inside the coreless linear motor easily rises due to the heat generated from the coil. As a result, positional deviation of components of the coreless linear motor occurs due to a temperature change. As a result, it is difficult to secure positioning precision of the coreless linear motor. From the viewpoint of heat radiation, preferably aluminum alloy or another metal having a high heat conductivity is used for the holding plate, but a metal having a high heat conductivity also has a low electrical resistance in comparison with stainless steel etc., therefore, when driving the linear motor, a considerably larger induction current than that in the case of use of stainless steel is generated. A force in a reverse direction to the thrust is generated due to the interaction of this induction current and the magnetic flux of the magnets and the fluctuation of the thrust with respect to the movable member in the linear motor becomes large. From the above viewpoint, aluminum alloy or another metal having a high heat conductivity is not suitable for use for the holding plate.    Patent Document 1: Japanese Patent Publication (A) No. 2002-165434